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concentration. The concentrations of numerous cations increased during [1972][1973][1974][1975][1976][1977][1978][1979][1980][1981][1982][1983][1984][1985][1986][1987][1988][1989][1990], as did the SO 4 2-concentrations. This variation is thought to be the result of a mechanism involving a second source, namely, volcanic emanations of a sulfur-rich composition, which were temporarily added to emanations from the first source; the first source has continuously supplied volcanic emanations to the original acid hydrothermal solution at Obuki Spring, and the second source is responsible for the increase in SO 4 2-, Cl -, and acidity. Variations in the concentrations of most other components can be explained as the result of changes in the interaction between the acid hydrothermal solution and the surrounding rocks; the latter changes were themselves the consequence of increases in acidity.

A small-scale steam explosion occurred on Karanuma crater on the summit of Akita-Yakeyama volcano on August 16, 1997 after a dormancy of 46 years. Chemical compositions of the fumarolic gases at the summit and hot spring waters around the volcano were monitored before the eruption.Obvious changes in the composition and outlet temperatures of the fumarolic gases were not detected, neither before nor after the 1997 eruption. Hydrogen and oxygen isotopic ratios of the gas condensates and hot-spring waters at the Yunuma crater indicated that a hydrothermal reservoir, where the fumarolic gases separated from the hot-spring waters at 150• C, existed in a shallow place beneath the crater. Smectite, kaolinite and pyrophyllite were identified in the clay fraction of the volcanic ejecta. Although pyrophyllite should have been formed at about 1 km beneath the summit, it was not directly derived from the deep zone during the 1997 eruption but had been ejected by previous eruptions. The Cl/S values of the water leachates of the ejecta were about 0.7, which indicated that the volcanic gas which caused the eruption was rich in HCl. However, the fumarolic gases and the water samples collected from the summit area contained little chloride. The source of the water-soluble chloride might be high-temperature magmatic gases that have been estimated as the source of Cl-SO 4 type thermal water. Such magmatic gases might have caused the 1997 eruption.

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